well, I don't remember exactly what level she is when I pick her up, but with doing only the quests I listed above she would usually be about 300k XP behind the rest of the party. that means about 1 level lower than the rest of my mages.
the scribing-scrolls-trick is definately a nice way to bring her level up and it also gives your PC a nice XP boost. if you're gonna use it, do it like this:
find a room with a door that can be closed. you and Imoen enter the room and close the door, the rest of the party are left outside. now reform your party and kick everyone except Imoen out; now you can do all the scroll-scribing you want without being interrupted by all the 'are you sure you want me to leave...?' crap. but make sure Imoen gulps down a couple Potions of Genius as in my games she has a nasty faillure rate of well over 50%! or just turn difficulty down to normal/novice.
once you're out of scrolls just open the door and let everyone know that you want them to remain with the groop. this will prevent your romance going down the drain; if you are in a romance anyway...